A polyarylene sulfide (PAS) resin as represented by polyphenylene sulfide (PPS) is an engineering plastic excellent in heat resistance, chemical resistance, non-flammability, electrical properties and mechanical properties, and is widely used as materials for fiber, films, injection molding and extrusion. The production of a PAS resin through a process of reacting an aromatic dihalide compound and an alkali metal sulfide in an organic polar solvent, such as N-methylpyrrolidone (NMP), is described, e.g., in Patent document 1 listed below. According to the process, a polymerization system in a high-temperature state is withdrawn by flushing into a vessel under a normal or reduced pressure, and is then subjected to recovery of the solvent, washing and drying to recover the resin, thereby finally obtaining a powdery resin product having particle sizes of from several μm to 50 μm. The powdery resin in a dry state is liable to be scattered as dust and is difficult to handle because of a small bulk density. Particularly, the powdery resin is liable to show inferior production efficiency because of a lowering in throughput at the time of melt-extrusion. Further, a PAS resin is generally provided with a level of melt-viscosity necessary for processing through so-called heat cross-linking in a step after the drying, so that the PAS resin is liable to exhibit inferior mechanical or physical properties than linear polymers.
Further, Patent document 2 below has disclosed a process for processing particulate PPS, and in its Examples, the polymerizate is discharged into water, and precipitated polymer beads are recovered and subjected to a repetition of washing with warm water.
For a similar resin, Patent document 3 below has disclosed a process for treating a polymerizate slurry obtained in the polymerization step and containing a particulate PAS. The treating process includes: (1) a step wherein a polymerizate slurry containing polyarylene sulfide particles, by-produced crystalline and dissolved alkali chloride and arylene sulfide oligomers together with the liquid component principally comprising N-methylpyrrolidone, is subjected to sieving for separation into polyarylene sulfide particles and a slurry containing crystallized alkali chloride, (2) a step wherein the slurry containing crystallized alkali chloride is subjected to solid-liquid separation to recover crystallized alkali chloride, while the liquid component is distilled to recover N-methylpyrrolidone, (3) a step of washing the polyarylene sulfide particles with an organic solvent, such as acetone, and water, and (4) a step of distilling the organic solvent washing liquid to recover the organic solvent. Patent document 3 does not disclose the detail of the washing step, whereas it is ostensible to construe that a batchwise washing and filtration operation is intended that requires repetitive washing which leads to a large-size apparatus and an increased production cost.
Patent document 4 below has disclosed a process for washing polyarylene sulfide, wherein polyarylene sulfide particles recovered by sieving from a polymerizate slurry are successively washed with an organic solvent, such as acetone, having a lower boiling point than water and miscible with water and polar organic solvent in the polymerizate slurry, and then with water.
Further, Patent document 5 below discloses a process for washing polyarylene sulfide particles by subjecting polyarylene sulfide particles recovered by removing the solvent from the reaction liquid by means of a filter to a countercurrent contact with a washing liquid in a tubular apparatus having therein static mixing elements disposed successively in series. According to our study, however, a sufficient effect of washing polyarylene sulfide particles cannot be attained according to a countercurrent washing process utilizing such static mixing elements.
In any case, the above-mentioned conventional processes for producing polyarylene sulfide involve many problems arising from the fact that the polymerizate slurry resulting from the polymerization step include many components.
More specifically, in any of the above-mentioned processes, an objective polymerizate slurry containing polyarylene sulfide particles obtained by reaction of an alkali sulfide source and an aromatic dihalide compound in a polar organic solvent, contains not only polyarylene sulfide particles having an average particle size on the order of 200-2000 μm, as a principal object of recovery, but also a polar organic solvent, by-produced alkali metal salt fine particles having an average particle size on the order of 5-100 μm, dissolved alkali metal salt and arylene sulfide oligomer. Such a large number of components in the polymerizate slurry provide an essential difficulty in separation and recovery of product PAS particles, and particularly many problems are encountered in the above-mentioned separation-recovery process including the sieving or filtration as an essential step.
For example, the above-mentioned processes disclosed by Patent documents 3 and 4 both include a step of sieving the polymerizate slurry by using a sieve having a mesh-opening of ca.105 μm so as to recover PAS particles on the sieve while transferring the by-produced alkali metal salt fine particles to the filtrate liquid. It is inevitable to lose PAS particles having particle sizes passing through the sieve mesh-opening, and if the mesh-opening is decreased so as to reduce the loss, the separation between the PAS particles and the alkali metal salt fine particles becomes difficult.
In order to solve the above-mentioned problems, the present inventors, et al., have developed a process for producing polyarylene sulfide which has been rationalized from the process standpoint by omitting the sieving or filtration step that is the source of the above-mentioned problems of the process, subjecting the polymerizate slurry directly to countercurrent washing and repeating the countercurrent washing (Patent document 6 below).
Patent document 1: JP-B 52-12240
Patent document 2: JP-A 59-49232
Patent document 3: JP-A 61-255933
Patent document 4: JP-A 4-139215
Patent document 5: JP-A 3-86287
Patent document 6: WO 03/048231
The process for producing polyarylene sulfide of Patent document 6 above which has been rationalized from the process standpoint of view has faced a quality problem of increased ionic impurities, such as residual alkali, in the product polyarylene sulfide, and a problem of lowering in yield of the product polyarylene sulfide.